The present invention generally relates to alpha particle detectors, and, more specifically, to detectors capable of detecting alpha particles originating from a point distant from the detector and utilizing alternating currents.
In any area where radioactive materials are handled, it is imperative, both for the protection of personnel and to avoid contamination of the environment, to continuously monitor personnel, equipment, and clothing to prevent the release of radioactive contamination. Alpha contaminants, such as plutonium, are particularly difficult to detect because of the limited penetration of alpha particles in air. Alpha particles from typical contaminants travel no more than one inch in air. It is because of this characteristic that prior alpha detectors have been useful only when used in close proximity to the point of possible emission. Currently, alpha contamination cannot be detected if it originates in a space that is too small for insertion of a conventional monitor. Prior detectors have normally been employed in personnel screening when moved slowly in close proximity to a person's body. Workers in nuclear processing facilities must place their hands and feet on sensors when moving from room to room. All of this can slow operations, as it is not currently possible to adequately screen personnel within a reasonable period of time.
It is also extremely difficult to monitor equipment for alpha contamination, again due to the limited range of alpha particles in air. Because of the monitoring difficulty, equipment that has been used in a potentially contaminated area is often classified as potentially contaminated and its further use is restricted to other controlled areas. If such equipment could be effectively monitored for contamination, the equipment could be released for use in uncontrolled areas. Previously, contamination inside assemblies has been impossible to detect without dismantling the assembly. The present invention allows contamination detection in any area that air can penetrate.
As used herein, the terms "long range," or "long distance," when referring to the detection capabilities of the present invention, shall mean detection from a range or distance of more than one (1) inch from the source of alpha radiation.
In the past, several instrument designs have been utilized to detect alpha radiation. Among these are GM tubes, ionization chambers, count rate detectors, and scintillation or gas flow proportional probes. While these instruments are capable of detecting alpha particles, they do so by directly detecting incident radiation, and must be within an inch of the source of the radiation. Also, these conventional alpha particle detectors can only scan an area approximately equal to the size of the detector.
The primary reason for an alpha particle's short flight path in air is its collision with air molecules. In almost all of these collisions, air ions are created which will have a longer life and area of influence than the alpha particles that created them. It is these ions that are detected by the present invention. The fact that the air ions have a longer range than the alpha particles relieves the necessity for having a detector moved over a person or equipment in order to detect the presence of alpha radiation.
In a copending application, Ser. No. 709,566, filed Jun. 3, 1991, a long range alpha detector is disclosed in which at least two grids are utilized. Another copending application, Ser. No. 07/773,002, filed Oct. 8, 1992, discloses single and double grid long range alpha detectors. The present application provides alternating current long range alpha detector. Both of these copending applications involve direct current detectors, that is, the flow of ions into the detector is continuous, and results in a direct current which must be amplified and measured. The use of alternating currents in the present invention allows the use of simpler and less expensive ac amplifiers, and avoids many dc error components. It is therefore an object of the present invention to provide apparatus for the long range detection of alpha particles utilizing an alternating current flow of ions to the detector.
It is another object of the present invention to provide apparatus for the long range detection of alpha particles providing an alternating current voltage to the grids.
It is another object of the present invention to provide apparatus for the long range detection of alpha particles which is less susceptible to error components than are dc detectors.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.